1. Field of the Invention
The present invention relates to an image processing apparatus that receives image data and performs a printing process and to an image processing method, and more particularly to an image processing apparatus provided with calibration control for stabilizing the coloration of a printed image and to an image processing method.
2. Description of the Related Art
In recent years, there has been increasing demand for direct imaging printers that do not require a plate used in offset printing and the like. Direct imaging printers are widely used because of the desire to shorten the printing time and to print a large number of copies, and because of environmental issues such as the disposal of paper arising from printing failures. Of these, inkjet printers that are cost effective and suitable for printing photos and electrophotographic printers that have high productivity and a finish approaching that of offset prints are most widely used. In such a context, the color stability of images formed on paper is one of the most important functions required of direct image printers as an alternative to conventional offset prints and photographs.
Techniques relating to color stability control have been previously proposed, in order to secure color stability. As one example of color stability control in an electrophotographic image processing apparatus, firstly a patch pattern obtained by performing halftone processing at a prescribed density level on a plurality of points are formed in the apparatus on an intermediate transfer member as a toner image, and, similarly, the densities of these patches are measured using a sensor provided in the apparatus. Next, the density characteristics of the halftone processing with respective to input density levels are computed based on these measurement results, and a density correction table is generated such that the input density levels in the print data will be prescribed standard density values. Subsequently, the input density levels of the print data are corrected using this density correction table. The density and coloration of print output can thereby be constantly maintained in a fixed range according to the input density levels (e.g., see Japanese Patent Laid-Open No. 2000-238341).
However, density measurement based on toner images on an intermediate transfer member is only able to obtain density information prior to toner images being fixed on paper, even though forming and erasing toner images is simple. For this reason, in the case where density control based on toner images is performed, the effect of the fixing process and processes downstream thereof cannot be reflected in the density control. In view of this, a method has been proposed whereby, in a copier, an image on output paper obtained by forming an image with a copier body (printer unit) is read using a reader unit set up in the printer unit, and image control is performed based on the read result, for example (e.g., see Japanese Patent Laid-Open No. 62-296669). However, with this method, the operations are troublesome, since the user must remove the output paper that has undergone image forming by the printer unit from a paper discharge unit, set output paper in the reader unit, and configure the image reading settings. For this reason, image control cannot be executed frequently, making it difficult to sufficiently stabilize image quality with respect to image characteristics of the image processing apparatus that are constantly changing. Techniques that involve setting up an optical sensor on a conveyance path downstream of a fixer for fixing a toner image on paper and detecting output images formed on paper have been proposed in order to solve the problems referred to above (e.g., see Japanese Patent Laid-Open No. 10-193689). However, in the case where sensors are installed on the conveyance path downstream of the fixer, disposing the sensors to span the entire area in a direction perpendicular to the conveyance direction is difficult in terms of cost and space, and realistically 1 to 4 sensors are disposed in most cases. For this reason, in the case where it is desired to detect a number of patches equivalent to control for reading an image with the above-mentioned reader unit, a patch pattern that is long in the conveyance direction needs to be formed, but if such a patch pattern is used, the detection time and the number of output sheets increase dramatically. Accordingly, the number of patches needs to be limited in the case where sensors are set up on the conveyance path, but when color stability control is performed on an output image with a small number of patches, achieving color stability with high accuracy with respect to all of the colors output by the printer is difficult.
Also, with the electrophotographic method, image reproducibility is constantly changing, since the output state fluctuates during the printing process due to various factors. Accordingly, even if the correction process is performed immediately before the printing process on image data, in cases such as where the number of pages of the image data to be printed is very large, the processing time needed for printing will be lengthened, and the output characteristics will fluctuate greatly. As a result, the coloration of specific colors may vary during the printing process on the same image data.